PSI - Issue 33

Paolo Ferro et al. / Procedia Structural Integrity 33 (2021) 198–206 P. Ferro et al./ Structural Integrity Procedia 00 (2019) 000–000

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2.2. Heat treatment simulation After welding simulation, the joint underwent the furnace thermal history used to carry out the experimental stress relief heat treatment. In the mechanical computation the viscous plastic deformation (  p ) of the alloy was modeled by using the following creep law (kinematic approach):

(2)

(3)

where  is the kinematic strain-hardening variable and H, K, C, P and n are temperature dependent constants (Table 5). The term describes the strain hardening and  (3/ 2) P  1 C  P corresponds to viscous recovery. Primary creep corresponds to the commencement of the experiment, when α has reached its static state (obtained by cancelling in equation (3), the value of being less than that for primary creep). Secondary creep therefore results, in this model, from a state of equilibrium between strain hardening and viscous recovery. This corresponds to physical reality.

Table 5. Creep law parameters as a function of Temperature

log 10 (K)

n 1

H

log 10 (C)

P 1 4 7

T [  C]

400 482 565 650

-80

10

-1

-15.555

3.4

85000 29040 12000

-12.511 -15.511 -6.2218

-15

4.11

-11.523

3.2

1.9

3. Results and discussion 3.1. Thermo-metallurgical results

Fig. 4 shows some results of the temperature computation. The fusion zone dimension and shape are in sufficient good agreement with that obtained in the experiments. Metallurgical results and comparinon with experiments are summarized in Fig. 5. The FZ microstructure consists of acicular and Widmanstätten ferrite with islands of allotriomorphic ferrite, in agreement with the microstructure observed in low carbon steels welded joints (Babu, 2004). In the range between 800-300 °C the austenite decomposes to different ferrite morphologies. The austenite to ferrite decomposition starts with the formation of allotriomorphic ferrite at the prior  boundaries. With continued cooling, the Widmanstätten ferrite nuclease at ferrite/austenite boundaries and extends into the untransformed austenite-grain interiors. With further cooling to low temperatures, the acicular ferrite would nucleate on the inclusion. If there are no inclusions, bainitic ferrite might form instead of acicular ferrite, from the remaining austenite. The heat affected zone (HAZ) of the as-welded joint was mainly composed of bainite.